Quadratic Solitons

Lluis Torner and Anatoly P. Sukhorukov

The experimental demonstration in 1995 of spatial soliton formation mediated by parametric wave interactions in crystals with quadratic nonlinearities, which had been theoretically predicted in the 1970s, opened a range of exciting new opportunities in the field of soliton science and its applications. For many years, second-order (or quadratic) nonlinearities had been associated only with the frequency conversion of laser light. A crucial step towards richer opportunities was the emergence of cascading, where cross-induced energy- and phase-shifts acquired by multiple light waves that parametrically interact in a material with a quadratic nonlinearity are exploited to perform all-optical operations on signals. To form a soliton, or in other words a self-sustained, localized, non-spreading light packet, such energy- and phase-shifts can also be used to dynamically counteract spreading caused by diffraction and by group-velocity dispersion.